Positioning is a technology of finding out a position, a speed, and the like and a current wireless positioning technology is being developed to utilize the existing network, utilize a new independent network, and utilize a global positioning system (GPS). In connection with the technology development trend, a general method of performing wireless positioning may be classified into a network-based method, a handset-based method, a dedicated network-based method, and a method using a GPS.
The positioning method may be generally classified as follows. There are an angle of arrival (AOA) method of obtaining a position of a handset by measuring an angle of arrival of a signal incoming from a base station to the handset, a method of using time of arrival (TOA) obtaining a position by measuring a radio propagation time based on a method of using a time of arrival of a radio wave, and a time difference of arrival (TDOA) method of using a relative difference between times of arrival of radio waves from two base stations. A method of finding out a position using a signal from a GPS satellite may be a representative TOA method.
The TOA method is a method of obtaining a distance by measuring the radio propagation time between the handset and the base station. The principle is that circles are generated based on each base station from several measurement values measured by several base stations and the handset is put at cross points of the circles.
Basically, in the case of the method of using a TOA, both of the base station and the handset need to be accurately synchronized and time-stamped to allow the base station to find out when the signal from the handset starts needs to be performed. In the positioning system using the TOA method, there is a need to know positions of at least four base stations and a pseudo range up to each base station to obtain a position of the handset. If an altitude is fixed, there is a need to know positions of at least three base stations.
In the system using the GPS satellite, a recursive least square is generally used to determine an absolute position. Uniqueness of the obtained solution is verified and a method of analytically obtaining a solution without the recursive least square has been also proposed. However, a terrestrial wireless positioning system may have a relatively smaller pseudo range than the system using the GPS satellite and frequently cause the situation in which positions of receivers of the base station receiving the signal from the handset are substantially the same plane.
In the situation, a dilution of precision (DOP) is poor, and therefore even though the measurement value is accurate, position accuracy is largely reduced, in particular, accuracy in a vertical direction is largely reduced. When the solution is obtained based on a small pseudo range measurement value, several solutions satisfying a navigation equation may be present. To solve the above problem, a direct solution method of directly obtaining a solution without recursion may be used.
The TDOA method determines a position based on the time difference of arrival of signal transmitted from different places. The time difference of arrival of a radio wave in proportion to a distance difference from two base stations to the handset is measured and the handset is positioned on a hyperbola using a place at which a distance difference from the two base stations is constant, that is, the two base stations as a focus.
Two hyperbolas are obtained from three base stations and the handset is positioned at a cross point between the two hyperbolas. The TDOA is generally measured based on a cross correlation method.
The TDOA method does not measure absolute time information but measures only the received time difference to be more easily implemented than the TOA.
In the positioning system as described above, the time when the signals transmitted from the target are transferred to each receiver needs to be accurately measured. The precision of the time measurement depends on the reference clock used in a receiver system. That is, as the reference clock is fast, an input time of the target signal may be measured more precisely. However, as the reference clock speed is increased, the amount of data for the input signal to be processed may be increased.
For example, a multilateration (MLAT) system mainly uses a clock of 100 MHz to generate a clock at 10 ns. In this case, a maximum time error may be 10 ns and a distance error thereof may be 3 m.
Korean Patent Laid-Open Publication No. 10-2007-0117408 discloses a multi-purpose reaction measuring system and method.